1 Field of the Invention
The present invention relates generally to the fabrication of photomasks from which patterns may be transferred to semiconductor device structures. More particularly, the present invention relates to a method for adjusting dimensions of photomask features, such as so-called “critical dimensions, ” subsequent to etching a desired pattern therein and prior to utilizing the photomask to transfer the desired pattern to a semiconductor substrate.
2. State of the Art
Reticles, or photomasks, are often used in the semiconductor industry as templates for creating desired patterns in semiconductor substrates. Photomasks are typically comprised of a silicon oxide-containing substrate (e.g., glass or quartz) having a chrome-containing layer on one side thereof in which a pattern is etched. When aligned with a surface of a semiconductor substrate (conventionally with a layer of photoresist material therebetween), photomasks may be used to transfer the pattern from the photomask to the surface of the semiconductor substrate. Photomasks are typically used in place of directly writing the desired pattern on the semiconductor substrate as a substantial amount of time and expense may be saved by blanket processing through a photomask.
A photomask may be fabricated using a number of different techniques, depending upon the method of pattern writing utilized. Due to the dimensional requirements of modern semiconductor structures, writing of features on a photomask is typically conducted with a laser or electron beam. A typical process for fabricating a photomask is illustrated in FIGS. 1A–1D. It should be understood by those of ordinary skill in the art that the methods and structures described herein do not form a complete process for manufacturing photomasks. The remainder of the process is known to those of ordinary skill in the art and, therefore, only the process steps and structures necessary to understand the typical fabrication process are described herein.
Referring to FIG. 1A, a cross-sectional view of an intermediate structure 10 in the fabrication of a photomask 20 (see, FIG. 1D) having a pattern etched in the chrome-containing layer 14 thereof is illustrated. The intermediate structure 10 includes a chrome-containing layer 14 which resides on a silicon oxide-containing substrate 12, such as a glass or quartz plate. The figures presented in conjunction with this description are not meant to be actual cross-sectional views of any particular portion of an actual prior art photomask, but are merely idealized representations which are employed to more clearly and fully depict the process of the prior art than would otherwise be possible. Elements common between the figures maintain the same numeric designation.
An anti-reflective coating (ARC) layer 16 is disposed over the chrome-containing layer 14. The ARC layer 16 may be an inorganic ARC layer formed, for instance, from chrome oxynitride, titanium nitride, or silicon nitride; an organic ARC layer formed, for instance, from poly(vinyl pyridine), or polyimide; or a combination of inorganic and organic materials. A photoresist layer 18, formed from a conventional photoresist material, is disposed atop the ARC layer 16.
As shown in FIG. 1B, after formation of the intermediate structure 10, a desired pattern of features is directly written on the photoresist layer 18 (e.g., using a laser or electron beam) and the pattern is developed into the photoresist layer 18, creating a patterned photoresist layer 18′. The patterned photoresist layer 18′ is subsequently etched to extend the pattern into the unprotected portions of the ARC layer 16 and the chrome-containing layer 14, as shown in FIG. 1C. Etching processes are known to those of ordinary skill in the art and may include, without limitation, reactive ion etching (RIE). As shown in FIG. 1D, the patterned photoresist layer 18′ may subsequently be removed by a conventional process (such as a wet- or dry-strip process, a tape lift-off technique, or combinations thereof. Thus, a photomask 20 having a desired pattern in the chrome-containing layer 14 thereof is fabricated. The ARC layer 16 typically remains over the chrome-containing layer 14 to reduce reflection and make any defects that may be present more visible, such potential defects being more fully described below.
For a variety of reasons, some photoresist layers used to fabricate photomasks according to the above process, as well as other conventional processes, may contain defective patterns. For instance, referring to FIG. 2A, undeveloped areas may occur at the base of the patterned photoresist layer 18′ causing the presence of a foot or feet 22. Feet are thought to be caused by the neutralization of an acid in the patterned photoresist layer 18′ by the basicity of the underlying ARC layer 16. Further, t-topping may occur wherein a portion of the photoresist material 24 at or near the top surface 26 of the patterned photoresist layer 18′ extends further than desired in the lateral direction. T-topping is believed to be caused by surface contamination of the photoresist material 24 during processing prior to developing the pattern into the patterned photoresist layer 18′. Still further, standing waves 28 may occur on the sidewalls 30 of the patterned photoresist layer 18′ due, it is thought, to reflectivity of the underlying chrome-containing layer 14 during the writing process, despite the presence of the ARC layer 16.
As shown in FIG. 2B, if a patterned photoresist layer 18′ having a defective pattern is used to fabricate a photomask 20, the defective pattern will be transferred to the photomask 20 and may, accordingly, affect one or more critical dimensions thereof. A “critical dimension” in a photomask is the size of any feature of the photomask which is critical to performance of a semiconductor device patterned through the photomask. Often, but not always, the critical dimension is the size of the smallest feature on the photomask. Typically there is some tolerance, i.e., differing upper and lower limits, for the allowable size of a particular critical dimension. However, such tolerances are generally rather narrow. In the intermediate structure 32 of FIG. 2B, the critical dimension is shown to be the lateral width of line 34, such lateral width designated “x.” However, if one were to etch through the patterned photoresist layer 18′ of FIG. 2B, which contains a defective pattern, the lateral width of line 34 would instead be represented by “x′,” which is less than “x” and may be outside of the critical dimension tolerance. In a case such as this, the resulting photomask 20 conventionally must be discarded as it does not meet the critical dimension requirements. As is apparent, this results in a significant cost.
Methods for fabricating photomasks having critical dimensions within the critical dimension tolerance, despite defects in patterned photoresist layers, have been proposed. For example, U.S. Patent Publication No. U.S. 2002/0160274 discloses a method for improving control over the dimensions of the patterned photoresist by treating the patterned photoresist with an etchant plasma to reshape the surface thereof prior to etching through the photoresist to the chrome-containing layer. However, the inventors hereof are unaware of any methods for adjusting critical dimensions of a photomask once a defect has been extended to the chrome-containing layer thereof.